An existing driving circuit for a pixel unit is shown in FIG. 1. Such driving circuit comprises 2 transistors and a capacitor, wherein one transistor is a switching transistor T1, controlled by a scanning signal Vscan output from a scan line, for controlling an input of a data signal Vdata on a data line, and another transistor is a driving transistor T2 controlling light emission of an OLED; Cs is a storage capacitor for maintain a voltage applied to the driving transistor T2 during non-scanning period. The above circuit is referred to as a 2T1C driving circuit for a pixel unit.
An AMOLED is driven by a current that is generated by a driving transistor in saturation state to emit light. Because when the same gray scale voltage is input, different threshold voltages of the driving transistor lead to different driving currents, causing inconsistencies of the currents. During the manufacturing process of a Low Temperature Polycrystalline Silicon (LTPS), the uniformity of threshold voltage Vth is very bad, and at the same time the Vth drifts as well, therefore, the brightness uniformity of the traditional 2T1C driving circuit for a pixel unit has always been very bad.